Antistructure agent

ABSTRACT

An antistructure agent consisting essentially of hexamethyltrisiloxanediol and methoxyhexamethyltrisiloxanol is prepared by reacting hexamethylcyclotrisiloxane with water, methanol and a short-chain fatty acid. It is very effective in preventing structure buildup in silicone elastomers.

United States Patent 1 Lewis, Richard Newton ANTISTRUCTURE AGENTInventor: Lewis, Richard Newton, Tecumseh,

Mich.

Assignee: SWS Silicones Corporation, Adrian,

Mich.

Filed: June 20, 1974 Appl. No.: 481,006

US. Cl. 260l29.l SB; 260/37 SB; 260/465 R;

Int. Cl. C08G 77/04 Field of Search 260/465 R, 46.5 G, 37 SB, 260/29.lSB

[ 1 Dec. 9, 1975 [56] References Cited UNITED STATES PATENTS 3,328,3406/]967 Vaughn, Jr. 260/465 G 3,551,382 12/1970 Schnurrbusch et a1.260/465 G Primary ExaminerMelvyn l. Marquis [57] ABSTRACT 2 Claims, NoDrawings ANTISTRUCTURE AGENT Short-chain siloxanediols are often used asantistructure agents or softeners" in silica-filled silicone elastomers.They associate strongly with the filler particles and thus prevent orminimize association of the silicon polymer with the filler, whichotherwise leads to a gradual buildup of structure" or crepehardening."Structure" in such a case is a three-dimensional network that interfereswith easy processing of uncured elastomers, and is very undesirable.

Compounds which have been used as softeners are alphaomega-siloxanediolsof the general formula HO[(CH SiO],H, in which x is in the range of fromto 20. Also, alpha-alkoxy-omega-siloxanols of the formula RO[(CH),SiO],,H, where R is a short-chain alkyl radical and y is in the rangeof 3 to 5, are also effective as antistructure agents (see US. Pat. No.3,799,962). The effectiveness of these antistructure agents increases asthe hydroxyl content increases, i.e., as the chain length in eitherseries decreases, Unfortunately, very short-chain siloxanediols aredifficult to obtain.

Therefore, it is an object of this invention to provide a method forproducing a mixture of short-chain alphaomega-siloxanediols andshort-chain alpha-alkoxyomega-siloxanols. Another object of thisinvention is to provide a method for producing a mixture consistingessentially of hexamethyltrisiloxane-l ,5 -diol andlmethoxy-hexamethyl-trisiloxane-S-ol. A further object of this inventionis to provide a novel antistructure agent that is effective at lowconcentrations in a silicafilled silicone elastomer.

These objects and others are achieved, generally speaking, byhydrolyzing hexamethylcyclotrisiloxane (D with water in the presence ofmethanol and a weak acid catalyst, if desired.

Hydrolysis of D;, has not been practical in the past because water and Dare not appreciably miscible, even at temperatures above the meltingpoint of D which is about 65C. Hydrolysis under drastic conditions,e.g., with strong acids, or with water heated under pressure above 100C.may be accomplished, but invariably leads to a polymer with a lowhydroxyl content. Comparable results are obtained if a mutual solvent oflow to medium polarity, such as acetone or tetrahydrofuran, is employed.

Methanol, a solvent of high dielectric constant, unexpectedly permitshydrolysis to proceed readily under mild conditions, with relativelylittle involvement of the methanol. Even if a large molar excess ofmethanol is employed to assure complete homogeneity, the propor tion ofsiloxanediol produced is usually at least equal to that of themethoxysiloxanol. The trisiloxanediol, HOD l-l, is more desirable thanthe methoxytrisiloxanol, CH OD H, since it has a higher hydroxyl content14.2 percent vs. 6.7 percent). The methoxy group may have some effect,but it is less than that of the OH group. [n the above formulas Drepresents the dimethylsiloxane unit, (CH ),SiO.

It is preferred that the reaction mixture be homogeneous; otherwise thewater tends to concentrate in one phase and the D in the other phase.This leads to undesirable side reactions, notably polymerization of theD e.g.,

The mole ratio of water to D should be about 1:1 although somewhat loweror higher ratios are permissible. The preferred range is from 0.5 to 2.0moles of water per mole of D Less water gives relatively little of thedesired HOD l-l. More than 2 moles may be used but requires an excessiveamount of methanol to maintain homogeneity.

The amount of methanol is not critical and may vary over a wide range.For example, one mole of water and one mole of D require about 5.7 molesof methanol to provide a clear solution. The D melts and dissolvescompletely on heating to about 55C. A little less than 5.7 moles ofmethanol may be used satisfactorily; with good agitation the mixturebecomes homogeneous as the reaction progresses. However, it is preferredto use at least 3 moles of methanol per mole of D or 3 moles per mole ofwater, whichever is greater. A mixture of 5 moles of methanol to one ofD and one of water is very satisfactory. Larger amounts of methanol maybe used but are not usually desired unless required to dissolveimpurities in the D In any case it is preferred to use no more thanabout 10 moles of methanol per mole of D or water, whichever is greater.

'Although best results are obtained with D free of impurities, someimpurities may be tolerated. The principal impurities found inindustrially produced D is octamethylcyclotetrasiloxane (D For practicalpurposes D is an inert impurity, as its rate of hydrolysis is much lowerthan that of D Good results are obtained with D which contains novisible liquid at room temperature. This corresponds to a level of nomore than about 20 percent of occluded D Although good results have beenobtained with a slush of D containing up to 50 percent of D.,, thehigher amounts of D are undesirable.

An undesirable impurity is a polydimethylsiloxane produced on prolongedstorage of D by the action of traces of acid or alkali. This shows up inthe finished product as a viscous oil that floats on the surface.Preferably there should be less than one percent of this polymer in theD The hydrolysis is best carried out at a temperature at which all ofthe D is in solution. Generally, this requires a minimum temperature offrom 50 to 60C. unless a large volume of methanol is employed. Highertemperatures, such as the reflux temperature, have been used withsatisfactory results. Still higher temperatures may be used if desired,e.g., to achieve a rapid reaction without a catalyst. Pressure vesselsare then required and temperatures as high as C. or even C. may then beused.

Generally, the reaction proceeds slowly without a catalyst. The reactionrate may be increased considerably by the use of selected acids andbases; however, strong acids and strong bases are not recommended asthey promote side reactions such as condensation and equilibration. Weakacids, i.e., those having pK values between 1.5 and 6 are satisfactorycatalysts, provided they are removed when the desired reaction iscompleted or nearly completed. Weak bases may also be used, butgenerally they give more by-products than do weak acids. Water-solubleacids are particularly preferred as they are easily extracted at the endof the reaction. Among the acids that are suitable are the organic acidssuch as the lower fatty acids, e.g., formic acid and acetic acid;hydroxy acids such as glycolic and lactic; and polybasic acids such asmalonic, succinic, tartaric, and citric. Salts of weak bases may also be3 used as catalysts. Whatever catalyst is chosen, the concentrationshould be such as to give a pH between 2 and 5. Although the reactionwill proceed at pH values between and 7, it is slow in this region.Catalyst concentrations in the range of 0.00l to 5 percent are usuallyadequate.

It is sometimes found that one or more of the reagents is contaminatedwith strong acid. This may lead to condensation reactions during mixingand heating to reaction temperature, especially if a large volume ofmaterial is being handled. As a safeguard it is recommended that a smallamount of sodium bicarbonate be added to the methanol and water beforethe D is added; sodium bicarbonate has very little catalytic activity ofits own. Likewise, a small amount of a very weak acid may be added toneutralize alkali contamination. The catalyst is best added after theother ingredients have been heated to the desired temperature and the Dhas dissolved.

The reaction time depends on the temperature, the dielectric constant ofthe medium (which is determined by the amounts of water and methanol)and the amount and nature of the catalyst. Other conditions being keptconstant, reaction times may be varied between minutes and i0 hours bythe proper choice of catalyst.

It will be understood that, like other first-order reactions, thereaction is never 100 percent complete. Thus, if the half-life of D isl0 minutes, it will be half consumed in 10 minutes, consumed in minutes,''/a consumed in minutes, and so forth. It is preferred to carry out thereaction to at least 75 percent completion to prevent crystallization ofthe remaining D Maximum hydroxyl content is usually achieved after 85 to95 percent completion. Prolonged heating after that leads to a gradualdecline in hydroxyl content with relatively little change in D content,possibly because the reaction is to some extent reversible. The termreaction time as used here means the time required to consume about 90percent of the D At the end of the reaction period a volume of coldwater equal to about 150 percent of the volume of the original mixtureis added. This accomplishes several purposes. It cools the mixture, itstops the reaction by diluting and extracting the catalyst, and itextracts part of the methanol. When two distinct layers have formed thebottom layer is removed. Further extractions are carried out to removethe remaining methanol. Preferably the subsequent extractions are donewith solutions containing sodium chloride and sodium bicarbonate as thedensity of the product approaches that of pure water when the methanolis removed. The sodium bicarbonate helps remove traces of the remainingacid catalyst. Generally 3 or 4 extractions are all that is necessary toremove the methanol.

The product is a mobile liquid generally having between 5 and 11 percentof silicon-bonded OH and between 2 and 4 percent of silicon-bonded OCHas shown by nuclear magnetic resonance. The mole ratio of OH to OCH isgenerally between 2.5 and 7.0. The principal ingredients are HOD H andCH OD H with minor amounts of D and sometimes D D D and linear siloxanesof up to 6 silicon atoms, as shown by gas chromatography.

The products of this invention are effective and efficient antistructureagents for silica-filled silicone elastomers. These silicone elastomersare prepared in the conventional manner, namely by curing at elevatedtemperatures heat-curable organopolysiloxanes containing theseantistructure agents, vulcanizing agents, reinforcing and/ornonreinforcing fillers. The organopolysiloxanes useful in the inventionare commonly referred to as dialkyl or alkylaryl polysiloxane gums.These organopolysiloxanes are well known in the art and methods forproducing such materials are old and widely described in the literature.The curable organopolysiloxanes have recurring structural units of thegeneral formula where n is a number of from about 1.9 to 2.2 and Rrepresents monovalent hydrocarbon radicals such as alkyl, aryl, aralkyl,alkaryl, alkenyl, halogenated monovalent hydrocarbon radicals andcyano-substituted alkyl radicals. It is also desirable that in thecurable organopolysiloxanes the majority of the R radicals be loweralkyl radicals, for example, methyl radicals. It is usually preferredthat the organopolysiloxanes from which the curable compositions areprepared contain an average of from about L98 to about 2.2 organicgroups, for instance, methyl groups or methylphenyl groups, etc., persilicon atom and that more than 98 percent of the silicon atoms of thepolysiloxane contain two siliconbonded organic groups, for instance,alkyl groups or a mixture of alkyl and aryl groups, etc., per siliconatom. Included specifically in this formula are thedimethylpolysiloxanes, methylphenylpolysiloxanes,methylvinylpolysiloxanes, and copolymers of such units, such ascopolymers containing dimethyl-, diphenyl-, and phenylmethylsiloxaneunits and copolymers containing phenylmethyl-, dimethylandvinylmethylsiloxane units.

A sufficient amount of the above antistructure agents should be employedin these compositions to provide an OH content of at least 1 percent ofthe weight of the reinforcing filler. The amount required is more orless inversely proportional to the OH content. For example, if aformulation requires 18 parts of a standard softener of 2.5 percent OHit may be replaced with about 6 parts of a new softener having 7.5percent OH. The resulting product shows all the desirable properties ofthe earlier product, both before and after curing. In terms ofcompression set the new product is better because a lower compressionset is obtained with smaller proportions of softener. In general, theamount required is in the range of about 3 to about 12 parts per lOOparts of silicone gurn depending on the OH content of the softener andthe amount of high-surface silica used as a filler.

The new softeners may also be used to improve substandard softeners of,say, 1.5 to 2.0 percent OH by blending up to 2.5 percent or 3.5 percentOH. The blends are in every way equivalent to standard" softenerscontaining 2.5 percent or 3.5 percent OH.

Finely divided fillers such as reinforcing and nonreinforcing fillersmay be incorporated in the curable organopolysiloxane compositions. Theamount of fillers used in combination with the organopolysiloxanepolymers may be varied within wide limits, for instance, from about 10to 300 percent by weight of fillers based on the weight of theorganopolysiloxane polymers. The exact amount of fillers used willdepend upon such factors as, for instance, the application for which thecurable organopolysiloxane compositions are intended,

the type of fillers employed, the density of the fillers,

the type of curable organopolysiloxanes employed, etc. Obviously,mixtures of reinforcing fillers with nonreinforcing fillers may beemployed.

Examples of suitable fillers which may be used are asbestos, clay,hydrated calcium silicate, zinc sulfide, silica aerogel, fumed silica,barium titanate, glass fiber, floc, iron oxide, bentonite, zinc oxide,nickelous oxide, magnesium oxide, micronized graphite, micronized slate,micronized mica, celite, lead oxide, titanium dioxide and calciumcarbonate. Those having surface areas above 50 square meters per gramare generally classed as reinforcing fillers.

Various curing agents may be added to the organopolysiloxanecompositions to efi'ect rapid conversion of the compositions to anelastomeric state. Organic percent. The OH content was 6.56 percent andthe OCH content 3.54 percent, the mole ratio of OH to OCH being 3.28.Analysis by gas chromatography shows a variety of cyclic and linearsiloxanes mostly in the range of 3 to 6 silicon atoms of'which 28percent was HOD H and 21 percent was a mixture of HOD H and CH OD H; noother siloxane over 13 percent was present.

EXAMPLE 2 A rate study was conducted at 65C. using the same ratio ofingredients as in Example 1, except that a substantially pure grade ofD, was used. Samples were taken at intervals, extracted as before andanalyzed by gas chromatography. The results of the gas chromatographicanalysis are illustrated in the following table.

oxides are particularly effective. Among such peroxides may bementioned, for example, benzoyl peroxide, t-butyl perbenzoate,bis(2,4-dichlorobenzoyl) peroxide, dicumyl peroxide, dialkyl peroxides,such as di-tbutyl peroxide, etc. These curing agents may be present inamounts ranging from about 0.1 to as high as 4 to 8 percent by weight oreven more based on the weight of the organopolysiloxane polymers.

The manner in which the antistructure agents of the present inventionare utilized may be widely varied. The antistructure agents arepreferably mixed with the organopolysiloxane gum just prior to or duringthe addition of the reinforcing and nonreinforcing fillers. Curingagents and other additives such as dyes, pigments and flame retardants,may be added to the organopolysiloxane compositions during the millingoperation.

The embodiments of this invention are further illustrated by thefollowing examples in which all parts are by weight unless otherwisespecified.

EXAMPLE l To 480 parts (15 moles) of methanol, 54 parts (3 moles) ofwater and 0.1 part of sodium bicarbonate was added 666 parts (3 moles)of freshly distilled and still molten D containing about 10 percent of DThe mixture was heated with stirring to 58C. whereupon the D; thatcrystallized when first added dissolved. At this time 14 parts (0.3mole) of formic acid was added. The temperature was maintained at 58 to65C. for 60 minutes. The mixture was then cooled by adding 1,800 partsof cold water as quickly as possible. Stirring was continued for 10minutes, then the mixture was allowed to settle and the water was drawnoff. After two additional extractions with water containing 5 percent ofsodium chloride and 5 percent of sodium bicarbonate, the product waspractically neutral (pH 7 to 8), clear, and colorless. lts viscosity wasabout 10.6 centistokes and its specific gravity at C. was 0.973. Nuclearmagnetic resonance indicated that only a trace of methanol was present.Nonvolatile matter, as indicated by weight loss under vacuum at 100C.was only 13 per- EXAMPLE 3 A rate study similar to that of Example 2 wascarried out at 58C. The results are given below.

Reaction Time, OH,

Hours It is apparent from these examples that little further reaction ofthe D occurs after a half hour at either 58C. or 65C., but thatcondensation and equilibration reactions gradually reduce the OHcontent. The methoxy content also decreases but generally at a slowerrate.

EXAMPLE 4 A mixture of 222 parts (1 mole) of D 18 parts (1 mole) ofwater and parts (5 moles) of methanol was heated to reflux (68.5C.) and6 parts (0.1 mole) of acetic acid was added. Reflux and stirring werecontinued for 17 hours while the temperature rose slowly to 71 .2C. Then500 parts of water were added to stop the reaction. After 5 extractionswith 5 percent sodium chloride in water, the product was neutral andfree of methanol. It then contained 6.75 percent OH and 5.15 percent ofOCH the mole ratio of OH to OCH being 2.5.

EXAMPLE A formulation containing 100 parts of a polydimethylsiloxane gumcontaining 0.2 mole percent of methylvinylsiloxane, 40 parts of fumedsilica (Cab-O-Sil), 8 parts of the antistructure agent prepared inExample 1 and 1.4 parts of bis(2,4-dichlorobenzoyl) peroxide was milledfor 1 hour until it acquired a smooth, uniform consistency, molded at apress-cure temperature of l l5C. for 5 minutes at 600 psi and thenpostcured for 4 hours at 200C. in a circulating air oven.

EXAMPLE 6 in a comparison example, 8 parts of an antistructure agent(OH-terminated polydimethylpolysiloxane) having 2.5 percent OH wassubstituted for the antistructure agent of Example 1. The resultingcomposition was extremely difficult to mill and had a short shelf lifebefore refreshening was required, whereas the composition containing theantistructure agent of Example I required very little milling and had along shelf life. Also, the cured composition had a higher hardness thanthe cured composition containing the antistructure agent of Example l.

Although specific examples are mentioned and have been herein describedit is not intended to limit the invention solely thereto but to includeall the variations 8 and modifications falling within the spirit andscope of the appended claims.

I claim:

1. A heat curable organopolysiloxane composition composed of anorganopolysiloxane polymer, an organic peroxide curing agent, areinforcing filler and a sufficient amount of an antistructure agent toprovide an OH content of at least I percent of the weight of the filler,said antistructure agent comprising a mixture of hexamethyltrisiloxane-l,S-diol and l-methoxy-hexamethyltrisiloxane-S-ol, in which the hydroxylcontent is between 5 and l 1 percent, the methoxy content is between 2and 4 percent, and the mole ratio of hydroxyl to methoxy is between 2.5and 7.

2. The composition of claim 1 wherein the organopolysiloxane polymer hasrecurring structural units of the formula where R is selected from thegroup consisting of mono valent hydrocarbon radicals, halogenatedmonovalent hydrocarbon radicals and cyanoalkyl radicals and n is anumber of from about 1.9 to 2.2.

1. A HEAT CURABLE ORGANOPOLYSILOXANE COMPOSITION COMPOSED OF ANORGANOPOLYSILOXANE POLYMER, AN ORGANIC PEROXIDE CURING AGENT, AREINFORCING FILLER AND A SUFFICIENT AMOUNT OF AN ANTISTRUCTURE AGENT TOPROVIDE AN OH CONTENT OF AT LEAST 1 PERCENT OF THE WEIGHT OF THE FILLER,SAID ANTISTRUCTURE AGENT COMPRISING A MIXTURE OFHEXAMETHYLTRISILOXANE-1,5-DIOL AND 1-METHOXY-HEXAMETHYLTRISILOXANE-5-OL,IN WHICH THE HYDROXYL CONTENT IS BETWEEN 5 AND 11 PERCENT, THE METHOXYCONTENT IS BETWEEN 2 AND 4 PERCENT, AND THE MOLE RATIO OF HYDROXYL TOMETHOXY IS BETWEEN 2.5 AND
 7. 2. The composition of claim 1 wherein theorganopolysiloxane polymer has recurring structural unitS of the formula